Of course, change the FontSize of TextBlock is a low-cost operation which you will not even think about optimizing. However, there are some cases that you would perform a high-cost operation like aplying blurring effect to an Image, sending a HTTP request and so on. And in those cases frequent high-cost operations can result in slowing down the performance of your app or cause some unexpected issues.

So you may want to do some tricks to handle this issue: 500ms after your change to UI and the high-cost operation should be fired.